After attending the Columbia Law School, Cohn passed the bar exam at the age of twenty-one. He became an Assistant U.S. Attorney for the Southern district of New York state, and eventually would play a significant role in the prosecution of Ethel and Julius Rosenberg in 1951. Cohn's cross examination of Ethel Rosenberg's brother, David Greenglass, is seen as the central event in that trial, which lead to the Rosenbergs' conviction and execution.